The present invention relates to a neural stimulator, and more particularly to a cochlear prosthesis used to electrically stimulate the auditory nerve. Even more particularly, the invention relates to an improved process for mapping a signal level into a stimulation current level.
Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information reaches the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or the destruction of the hair cells in the cochlea which are needed to convert acoustic signals into auditory nerve impulses. These people are unable to derive any benefit from conventional hearing aid systems no matter how loud the acoustic stimulus is made. This is because their mechanism for converting sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, there is no way auditory nerve impulses can be generated directly from sounds.
To overcome sensorineural deafness, numerous Implantable Cochlear Stimulation (ICS) systems—or cochlear prosthesis—have been developed which seek to bypass the hair cells in the cochlea (the hair cells are located in the vicinity of the radially outer wall of the cochlea) by presenting electrical stimuli directly to the auditory nerve fibers, leading to the perception of sound in the brain and at least a partial restoration of hearing function. The common denominators in most of these cochlear prosthesis systems have been the implantation of electrodes into the cochlea, and a suitable external source of an electrical signal for the electrodes.
A cochlear prosthesis operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally convert acoustic energy into electrical activity in the nerve cells. In order to effectively stimulate the nerve cells, the electronic circuitry and the electrode array of the cochlear prosthesis perform the function of separating the acoustic signal into a number of parallel channels of information, each representing the intensity of a narrow frequency band within the acoustic spectrum. Ideally, the electrode array would convey each channel of information selectively to the subset of auditory nerve cells that normally transmitted signals within that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from high frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex, and ideally the entire length of the cochlea would be stimulated to provide a full frequency range of hearing. In practice, this ideal is not achieved, because of the anatomy of the cochlea, which decreases in diameter from the base to the apex, and because of variations in the cochlea structure that exist between patients. Because of these difficulties, known electrodes can at best be inserted to the second turn of the cochlea.
The signal provided to the electrode array is generated by a signal processing component of the Implantable Cochlear Stimulation (ICS) system. In known ICS systems, the acoustic signal is processed by a family of parallel bandpass filters. Then, the output of each bandpass filter is independently amplitude mapped into a simulation level, using a mapping consistent with normal perception. In known systems, the mapping is a compressive mapping that is based on the log of the magnitude of the individual outputs of the band pass filters. Due to the compressive nature of the mapping, frequency bins containing large signals are reduced in amplitude much more than frequency bins containing small signals.
Representative cochlear implant systems are disclosed in U.S. Pat. Nos. 5,603,726; 6,219,580; 6,289,247 and 6,308,101; each of which patents is incorporated herein by reference.
In known ICS systems, individual stimulation channels are not well isolated from their neighbors, due both to electrical and neural interactions within the cochlea. As a result, interaction occurs between neighboring stimulus channels. Further, when signals are separated into frequency bands by the parallel bandpass filter bank, and then compressed on an individual channel basis, there is a potential for spectral contrast reduction. The spectral contrast reduction results because the channel compression reduces the amplitude of higher amplitude sounds in one frequency band more than lower amplitude sounds in another frequency band, even if both of these sounds occur simultaneously. Disadvantageously, by narrowing the amplitude differences between weak information-carrying signals and strong information-carrying signals, the spectral contrast reduction, combined with the channel interaction, can reduce the user's ability to discern the information in the strong signals.
What is needed is a compressive mapping technique which reduces the degree of compression of large signals relative to smaller signals, while still allowing for a realistic mapping of overall loudness of sound from the acoustic domain to the electrical stimulation domain.